The present invention relates to a method for producing a processed milk containing galactooligosaccharide which is a bifidobacterium proliferation accelerating factor.
Galactooligosaccharide represented by a general formula Gal-(Gal)n-Glc (Gal represents galactose residue, Glc represents glucose residue and n represents an integer of 1 to 4)(referred to simply as galactooligosaccharide hereinafter) is one of major components of breast milk oligosaccharide, and is known also as a proliferation accelerating factor of bifidobacterium which is one of useful enterobacteria in human intestines. In recent years, therefore, attempts have been made to mass-produce galactooligosaccharide from lactose or from lactose-containing substances and to add the thus-produced galactooligosaccharide to milk products such as fermented milk and powdered baby milk.
Conventionally, galactooligosaccharide has been produced by, for example, a method in which lactose or a lactose-containing substance is processed by .beta.-galactosidase produced by Aspergillus oryzae. This method is disclosed in Japanese Patent Publication No. 58-20266 and Japanese Patent Unexamined Publication No. 60-41449. In the known production methods, the galactosyl transfer reaction, which has a priority to mere hydrolysis, takes place only when the lactose content is large. In fact, only hydrolysis takes place materially in processing a material having a lactose content of 10% or less such as ordinary unprocessed milk, skimmed milk and so on.
Therefore, conventional process for producing milk product containing galactooligosaccharide employs a steps of condensing or adding powdered milk to enhance the lactose content in the material milk and effecting an enzyme processing on the material milk to produce galactooligosaccharide from the lactose in the material milk. A method has also been used which employs steps of producing galactooligosaccharide by effecting an enzyme processing on a lactose solution of high lactose content and adding the thus formed galactooligosaccharide to a material milk.
These known methods, however, cannot provide satisfactory result when used in the production of a processed milk containing galactooligosaccharide mainly intended for use as a beverage. Namely, since the material milk such as an unprocessed milk or a skimmed milk cannot be processed directly, the material milk has to undergo condensation, enzyme-treatment and dilution so as to provide milk solid content optimum for drinking. This process is very complicated and, in addition, thermal transformation of milk protein is inevitable in this process.
A method also has been proposed in which galactooligosaccharide prepared from a lactose is added. Such a method, however, requires much cost and is inevitably accompanied by an excessive increase in the total sugar content of the product, resulting in unfavorable effects such as increase in the calorific value and increase in the degree of sweetness. This is because galactooligosaccharide industrially produced from lactose usually contain not only galactooligosaccharide but also di- and monosaccharides in amounts at least the same as that of galactooligosaccharide.